The present invention relates to a method for determining state information of a power semiconductor, the power semiconductor comprising a collector, an emitter and a gate, and a gate driver comprising auxiliary voltage input is connected to the gate of the power semiconductor.
High-speed semiconductors are applied in modern power electronic devices, such as frequency converters used for controlling the rotation speed of short-circuit motors, for instance. Typically, the semiconductors are IGB transistors, which generally tolerate a short-circuit for 10 microseconds at the most. Hence it is important that information on an overcurrent stressing the device is received as soon as possible, in order that power semiconductors can be switched to a non-conducting state before they are damaged as a result of overheating.
Saturation voltage of an IGB transistor, i.e. voltage across the collector and emitter of said component while the collector current flows, depends on the magnitude of the current passing through the semiconductor and its value on a normal operating range is 2 to 4 volts. Saturation voltage rises quickly, when the collector current exceeds the nominal current and so this fact is commonly used for detecting overcurrent. For instance in frequency converters, the maximum value of saturation voltage in direct short-circuit can be close to the voltage of the frequency converter's intermediate circuit. In general, the maximum value of saturation voltage depends on the short-circuit impedance. Commonly the detection limit is the voltage of 10 to 20 volts and the detection must continue for at least two microseconds without a break, in order that the situation would be regarded as an overcurrent that calls for action.
It is previously known to measure the saturation voltage of the power semiconductor in frequency converters by using, for instance, phase-specific and intermediate-circuit-specific voltage dividers with matched speed and amplitude against the negative bar of the frequency converter, and by comparing the value obtained by subtraction with the known reference value. The obtained digital data is in the negative bar potential, but it can be separated galvanically by using optoisolators, for instance.